I have a 408 W2 motor that now is in the 580HP range. The amount of money in the heads for purchase, porting and valve train is likely in the $4500 range. These still don't flow what a set of over the counter Apache heads do. Then there is the issue of block strength vs cost and availability. I know we still love our big blocks and LA engines, they dont hold a candle to the Gen 3 Hemi stuff on a cost per output and reliability basis.
I bet you could build a 6.4 with some drop ins, a cam, and head porting for $4500 and make 650HP easily. I drank the G3 hemi cool aide and I'm not going back. I still like the old school LAs but I won't ever build one again.